1. Field of the Invention
The present invention relates to a communication system having a plurality of exchanges each accommodating a plurality of terminals and a network for connecting these exchanges to each other for transfer of data among these terminals, more particularly relates to a method for switching communication modes in a communication system and an exchange suitable for the switching of communication modes.
In communication systems, the technology for transmission of data has been improved year by year in recent years. A further higher speed and more efficient transfer of data has become possible. On the other hand, for this purpose, work on developing exchanges is continuing, for example, from first mode exchanges to second mode exchanges and further to third mode exchanges.
As a result of such development work, all exchanges will ultimately be changed to the highest speed and highest efficiency third mode exchange.
During the transition, the exchanges of the first mode, second mode, and third mode will be mixed. In a communication system wherein exchanges of various modes are mixed in this way, however, it is not easy to change all of the exchanges of the first mode and second mode to exchanges of the third mode without hindering service to users transferring data.
The present invention relates to a technique for switching communication modes suitable for such a transition of exchange modes. Note that, as the exchanges of the first mode, second mode, and third mode, at the present time, there are specifically a packet exchange, a frame relay exchange, and an asynchronous transfer mode (ATM) exchange. The following explanation will be given of a frame relay exchange and an ATM exchange.
2. Description of the Related Art
In conventional communication systems, extensive use has been made of packet switch networks providing at the network side a function for the retransmission of data at the time of occurrence of error to enable high reliability data communication. Thereafter, however, the quality of dedicated lines provided by carriers, i.e., the quality of channels, was improved, so the necessity of providing the retransmission function at the network side abated. Under such conditions, by eliminating the need for the retransmission function and simplifying the communication processing by that amount, a frame relay switch network was developed enabling a further higher through-put.
Recently, there have been rising user demands for accommodating a data network comprised of a frame relay switch network and a general telephone network in one integrated network to enable the dedicated lines to be efficiently used. There has therefore been a tendency of a further shift to an ATM switch network employing a cell exchange system enabling a guarantee of bandwidth and a small delay in data communication considering the quality of service (QoS). The situation of this transition will be explained in detail later by referring to FIG. 22 to FIG. 24.
As will be explained later based on FIG. 22 to FIG. 24, at the time of the transition from the already existing frame relay switch networks (packet switch networks) to ATM switch networks, naturally there has been an accompanying transition in the format of data communications. Accordingly, it is necessary to minimize the downtime of communication accompanying the shift in order to keep the influence exerted upon the end user at the lowest limit. That is, the work for the shift must be finished in a short time. For this reason, the work for the shift such as replacement of hardware and change of connection of relay lines has to be kept extremely simple and smoothly carried out.
However, conventionally, it is necessary for each office to manually change the connections of relay lines with remote offices almost simultaneously, so there is a problem in that it becomes extremely difficult to quickly make the change in a short time when there are a large number of offices.
Also, in the work for reconnection of cables, which becomes necessary when changing connection of relay lines, it is extremely difficult from the viewpoint of the limited work time to completely eliminate human error such as connection error, so there is also a problem of a lack of reliability of the work for the shift.
Further, there are often various constraints on the installation sites. In addition, due in part to the lack of sufficient skilled human resources, there is a problem that execution of the work for the shift is accompanied by high risk in a large sized communication system that accommodates a large number of offices.